An internal protector is typically used in electromotive compressors to detect excess current that flows to the motor or to detect elevated ambient temperatures resulting from an abnormal operation or a constrained operation. Such a protector includes a thermally responsive bimetal element that responds to the excess current or elevated ambient temperature; it opens the circuit that supplies current to the motor on the occurrence of an overload operation or a constrained operation, thereby protecting the motor from damage due to burning or the like.
A fusite pin assembly (or an air-tight sealed terminal assembly) is provided in hermetic type electromotive compressors for the purpose of providing an interface with an external power supply source. The assembly includes a common terminal, a main coil terminal and a supplementary coil terminal and the internal protector is connected in series between the common terminal and the motor winding in the electromotive compressor.
FIG. 21 shows a cross section of a hermetic motor protector made according to the prior art. As shown in FIG. 21, protector 200 includes a housing 210 made of metal and a metal header 220 that mounts various component parts. The outer peripheral portion of header 220 is fixed and electrically connected to housing 210 and serves as a terminal. Header 220 has a hole at its center and a pin 221 is mounted in the hole electrically insulated from header 220 by means of a glass seal 222. Pin 221 is electrically connected to a stationary plate 230 in housing 210. Stationary plate 230 mounts one end of a snap-acting bimetal disc 231 by means of a weld slug 232. A movable contact 233 is disposed at the other end of bimetal disc 231 and the movable contact 233 is movable into engagement and out of engagement with a stationary contact 211 mounted on the wall of housing 210.
Pin 221 of protector 200 is connected to the common terminal of the electromotive compressor and housing 210 is electrically connected to the winding side of the motor. During normal operation of the electromotive compressor, electric current that is supplied from the common terminal to pin 221 flows to the motor coil through stationary plate 230, bimetal disc 231, movable contact 233, stationary contact 211 and housing 210. If, due to some reason whatsoever, the rotor of the motor of the electromotive compressor cannot rotate and an excess current (which will hereafter be referred to as the constrained current) flows to the rotor, heat is generated in the path described above and when it reaches the preset actuation temperature of bimetal disc 231, the disc snaps from one curved configuration to an opposite configuration and the movable contact 233 moves away from stationary contact 211, thereby opening the power source circuit. As a result, the motor of the electromotive compressor is protected from possible damage. Conventional motor protectors as described above have the following limitation: although such motor protectors have been very effective in protecting conventional equipment from any possible damage, improvements in the efficiency of the equipment to be protected in recent years has resulted in a decrease in the difference between the operating current during normal or rated operation and the constrained current which occurs during abnormal operation. As a result, operation of the equipment to be protected can be interrupted by the motor protector during times of rated operation. In other words, if a short-term excess load operation occurs in rated operation, operating efficiency of the equipment would be improved without causing deleterious affects if operation is not interrupted. Protector 200, shown in FIG. 21, is actuated by the heat generated by the current that flows to bimetal disc 231 and the ambient temperature that is transmitted to the bimetal disc. As the resistance of bimetal disc 231 is comparatively high, however, the amount of heat generated can be large even if the electric current is small. Because of this, the rated current (or the overload current at a time of a permissible overload operation) that can be caused to flow during a rated operation is restricted and, even during a permissible overload operation, there are cases where bimetal disc 231 is instantaneously actuated in snap action.
During a period of permissible overload operation, on the other hand, it would be desirable to control the generation of heat by the electrically conductive path including bimetal disc 231 and to discharge the heat that has been generated from components where it is not desired so as to prevent actuation of snap acting bimetal disc 231 during a period of permissible overload operation. In the case of the protector shown in FIG. 21, however, the stationary plate 230 that mounts bimetal 231, etc. is disposed away from housing 210, with a result that it is difficult to discharge the heat generated by the internal members such as the bimetal disc, etc. In addition, the conductive path L between pin 221 and bimetal 231 contributes to the generation of heat and this, too, lowers the electric current that goes to the protector.